Pyridine based single-site catalysts are known. See U.S. Pat. No. 5,637,660. These catalysts are particularly useful for making ultra-high molecular weight polyethylene (UHMWPE). See U.S. Pat. No. 6,265,504. Unlike the conventional UHMWPE made with the Ziegler catalysts, the single-site UHMWPE has narrow molecular weight distribution. The catalysts, however, have relatively low activity and the UHMWPE produced has relatively low bulk density.
Low catalyst activity means low efficiency and high cost of the polymer production. Similarly, low bulk density means low productivity per reactor unit. Polyethylene of low bulk density also dries slowly because it absorbs solvent and residual monomers. Further, low bulk density may result in inferior product quality.
New ethylene polymerization processes are needed. Ideally, the process would use the readily available pyridine based single-site catalysts, give high catalyst activity, and produce UHMWPE having increased bulk density.